In recent years, light-emitting organic components in the shape of organic light-emitting diodes (OLEDs) which emit coloured light, in particular white light, have attracted more and more interest. It is generally known that the technology of organic light-emitting components has great potential for possible applications within the held of lighting technology. By now, organic light-emitting diodes achieve performance efficiencies which lie within the range of conventional electric bulbs (cf. Forrest et al., Adv. Mat. 7 (2004), 624).
Organic light-emitting diodes are usually formed by means of a layer construction which is arranged on a substrate. In the layer construction, an organic layer array is arranged between an electrode and a counter electrode such that an electric potential can be supplied to the organic layer array over the electrode and the counter electrode. The organic layer array is made from organic materials and comprises a light-emitting region. Within the light-emitting region, charge carriers, namely electrons and holes, recombine which are injected into the organic layer array when applying the electric potential to the electrode and the counter electrode and are there transported to the light-emitting region. A substantial increase in efficiency during the light generation could be achieved by integrating electrically doped layers into the organic layer array.
Organic light-emitting components may be used in a wide variety of areas of application to generate light of any colour which includes in particular display devices, lighting devices and signalling devices.
In one embodiment, the organic light-emitting components can be designed in such a way that they emit white light. Such components have the potential to be a significant alternative to the lighting technologies currently dominating the market, for example incandescent lamps, halogen lamps, low-voltage fluorescent lamps or the like.
Nevertheless, substantial technical problems are still to be solved for a successful commercialisation of the technology of organic light-emitting components. In particular, it is a challenge to generate great quantities of light by means of OLED components, which quantities are needed for general lighting applications. The quantity of light emitted by an OLED component is determined by two factors. These are the light intensity in the region of the lighting area of the component and the size of the lighting area. The light intensity of an organic light-emitting component can not be increased at will. Furthermore, the life of organic components is also substantially influenced by the light intensity. If the light intensity of an OLED component is doubled, for example, its life is shortened by a factor of two to four. In this connection, life is defined as the time passing until the initial light intensity of the OLED component has dropped to half its value during operation with a constant current.
The lighting area of an OLED component for lighting applications has to be chosen in accordance with a desired emitted quantity of light. The aim should be for it to lie within the range of a few square centimetres up to a size of more than one square meter.
As an electrical component, OLED components are typically operated at a low voltage in the range of from about 2 V to about 20 V. The current flowing through the OLED component is determined by the lighting area. In the case of a relatively small lighting area of the OLED component of about 100 cm2, a current of 1 A would already be needed at an assumed current efficiency of 50 cd/A and an application light intensity of 5000 cd/m2.
To supply an organic light-emitting component with such a current, however, poses a significant technical problem which can not be solved without further ado in a cost-efficient manner in commercial lighting applications. As is known, the electric power loss of the current supply is proportional to the electric resistance of the feed and to the square of the flowing current. Thus, to keep the power loss small even at high currents, electrical feeds would have to be used with a very low resistance, i.e. a large cross-section. However, this is to be particularly avoided in a component whose outstanding property, amongst others, is the flat design. If larger component surfaces are required, the supply current would have to be increased further which would aggravate the problems in the current supply further.
For this reason, it was suggested to electrically connect several OLED elements within an organic light-emitting component in series (cf. GB 2 392 023 A). In this connection, the total surface area of the organic light-emitting component is divided into individual OLED components which are electrically linked with each other in one or more serial connections. In this way, the operating voltage of the light-emitting component is increased by a factor of about one which corresponds to the number of the OLED components connected in series, the flowing current being reduced by the same factor. By reducing the operating current while increasing the operating voltage, a marked simplification of the actuation of the light-emitting component may thus be achieved with the same performance as it is generally markedly easier to supply an electrical component with a high voltage instead of a high current. Another advantage resulting from the use of the serial connection of OLED components is that, in the case of a short between the two electrodes, namely the cathode and the anode, one of one of the OLED components, a part of the lighting area of the organic light-emitting component indeed fails, but all in all, the light-emitting component still emits light and the total quantity of light emitted even largely remains unchanged due to the now increased operating voltage for the remaining, not failed OLED components. Thus, such a light-emitting component with a serial connection of OLED components can still be used, even after a short of one of the OLED components. In contrast, an organic light-emitting component which only has a single OLED component is unusable in the case of a short between anode and cathode.
For the production of OLED light-emitting components with a serial connection of OLED components, however, a complex production method is necessary. On the one hand, it is necessary to structure the electrode which is formed on the supporting substrate to define the electrodes allocated to the individual OLED components connected in series. Furthermore, it is necessary to structure the organic layer arrays of the individual OLED components and the cover electrode formed thereon. For this, several known methods come into consideration.
In the case of OLEDs in which organic materials are used which may be applied by means of vacuum evaporations, a suitable method for the structuring is evaporation by means of shadow masks. Other methods are, for example, the application by means of LITI (“Laser Induced Thermal Imaging”) in which from a carrier film loaded with organic material, at least one part of the organic material is transferred onto the substrate in which the carrier film is heated by laser with pinpoint accuracy. However, the LITI method can only be used for the structuring of the organic layer array of the OLED components. To structure the cover electrode which itself usually consists of metals such as silver, aluminium or magnesium or a conductive transparent oxide such as indium tin oxide, another structuring method has to be employed.
The structuring methods result in considerable expense within the course of the production of the organic light-emitting component, resulting in high costs. In the case of the use of shadow masks, the problem of a limited resolution furthermore exists, i.e. the distance between the individual OLED components connected in series is limited by the dimensions of the bars of the shadow mask. In this connection, it should be noted that a certain bar width of the shadow mask is required, depending on the size of the recesses between the bars of the shadow mask, to ensure the mechanical stability of the shadow mask.
Thus, to simplify the structuring by means of shadow masks, it makes sense to dispense with a fine resolution of the regions structured by means of the shadow mask. This can be performed by designing the OLED components connected in series relatively large, for example having a size of about 1 cm2. Through this, it is made possible to use shadow masks with low precision which may be oriented by means of simple methods, for example by means of orientation using positioning pins. Such methods are markedly cheaper in mass production than methods for fine adjustment which are based on the orientation by means of positioning markers under a microscope, for example.
Furthermore, the use of shadow masks is a limiting factor in terms of the achievable processing times as a fine adjustment of the shadow masks plays a not negligible part in the total process duration. By means of using a less precise method, the process times associated with the positioning can be shortened.
For particular methods for the production of organic light-emitting components, for example the continuous roll-to-roll method, further problems result from the known use of shadow masks. On the one hand, the shadow mask has to be carried along with the substrate on which the layer stack with the electrodes and the organic layer array are to be formed in such a method without changing the position of the shadow mask in relation to the substrate. On the other hand, the shadow mask has to be aligned with the substrate in such a method, the substrate optionally having to be stopped in the roll-to-roll method. It is thus desirable to have a process available in which the use of high-resolving shadow masks is not required.
The use of less precise shadow masks does not really lead to an optimisation as it is associated with significant disadvantages. In this connection, only larger OLED sub-areas can be formed. If one of these sub-areas fails due to a short, a large part of the lighting area of the component becomes inactive, i.e. it remains dark during the operation of the light-emitting component. However, the entire component is significantly affected with regard to its functionality through this. In fact, there is a minor decrease of the voltage in a serial connection over the shorted OLED component through which the voltage for the other OLED components increases which is the reason why the transmitted light is all in all only changed slightly, however, the optical effect of the organic light-emitting component is substantially deteriorated. This is unacceptable for application purposes. The light-emitting component is perceived by the viewer as being faulty. Furthermore, electrical shorts in OLED components lead to the entire current which normally flows in a distributed manner over the entire surface area only being led through the point of the short. Through this, a high temperature rises takes place locally which results in ohmic losses and the risk existing that the resistance is markedly increased at the point of the short and the point of the short thus becomes isolating, for example due to a delamination of organic or inorganic layers. There is a risk that the encapsulation applied to protect the light-emitting component does not withstand this local thermal stress, in particular when using a thin-film encapsulation, as is considered nowadays for OLED lighting elements of the future. These disadvantageous effects increase all the more, the larger the surface area of the OLED component is.
Besides the efficiency and the life of light-emitting organic components, the appearance of the component above all is naturally critical for the commercial success. It is in the nature of the OLED that the radiation of light onto larger surface areas is not completely homogenous as the transparent electrode most notably has a relatively low conductivity of typically between 1 ohm/sq and 300 ohm/sq. This means that a voltage drop exists over the transparent electrode due to the current flowing through the OLED which results to certain areas of the OLD emitting darker light than others. Particularly striking is this problem in OLEDs with particularly steep current-voltage characteristics. This applies especially to OLEDs of the pin type which are considered as being particularly suitable for lighting applications due to their high performance efficiency. Different suggestions were made to improve the homogeneity of the OLED. So called metal grids, i.e. thin metal lines are widely used which are in contact with the transparent electrode and take over a big part of the current transport. This solution is technically elaborate as it is associated with several coating and photolithography steps.